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A Robust Fine Feature Printing Process



A Robust Fine Feature Printing Process
This paper evaluates many fine printing stencil factors that have an increased effect on the transfer efficiency of solder pastes.
Production Floor

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Authored By:


George Babka
Assembleon America, Inc., Alpharetta, GA, USA

David Sbiroli and Chris Anglin
Indium Corporation, Clinton, NY, USA

Richard Brooks
Christopher Associates, Kyle, TX, USA

Summary


With the introduction of 01005 chip components and 0.3 mm pitch CSP devices, electronic component packaging is pushing surface mount technology to the limits of its potential. Miniaturization is driving the electronics industry to implement the smallest and tightest pitch components in order to meet their customer demands.

But how much miniaturization is possible before there is a paradigm shift in the technology? At what point is solder paste no longer viable? How small of a feature can be printed with solder paste, and can this process be implemented into a production environment?


Most of the factors and critical parameters in ultra-fine pitch printing have been well understood and documented for over twenty years. Some of these parameters are squeegee speed, squeegee pressure, stencil design (technology, thickness & area ratio), and solder paste. But as the pitch and aperture sizes get smaller and smaller, we begin to see that additional factors start to have an increased effect on the solder paste deposition (transfer efficiency).

What are these factors and can we control them in order to obtain acceptable results for transfer efficiency and minimized variability? This paper will evaluate these additional factors and how they affect the transfer efficiency of the paste.

Conclusions


Throughout the years of solder paste printing, there are well documented reports on what the critical parameters are and how they impact solder paste deposition. These parameters are squeegee speed, squeegee pressure, and stencil design (technology, thickness, and aperture size). The goal of these experiments was to see if other factors have an increased effect on the transfer efficiency as the pad sizes and pitch get smaller.

Our results have shown the following:

1. The board set-up is a critical parameter in printing fine features. Specifically, the board gasketing is improved dramatically by holding the board flat without any topside clamps during the print operation. Additionally, supporting the stencil along the entire length of the squeegee blade improves print definition and transfer efficiency even further,

2. A fast separation speed can have a significant positive effect on reducing transfer efficiency variation. Fine feature pads that are located near the end of the board (close to the rails) may need further investigation on the effect of the separation speed.

3. As the pad size and spacing are decreased, the squeegee angle has a greater effect on transfer efficiency and the consistency of the print deposition.

4. Solder mask in between ultra-fine pitch apertures can be a significant source of variation in the overall transfer efficiency. The effect of solder mask needs to be studied further to draw accurate recommendations for pad design.

5. Lastly, since most printers will not allow the variation of the squeegee angle as a process parameter, until now we have not been able to test the effects of blade angle on process variation throughout the course of the day. Having the ability to change the angle during a print run can offer the end user significantly greater consistency

when faced with events such as a stencil wipe, changeover, or pauses in the print cycle.
This analysis has shown that in studying ultra-fine feature printing, we have revealed several new print parameters that can contribute to the process variation. Previously, these additional parameters have not had a large effect on the printing of standard devices used in the industry today.

However, as the pitch and stencil aperture openings decrease, the variables of squeegee blade angle and squeegee speed become an important factor in minimizing insufficient prints and maintaining low print variation.

For an ultra-fine feature assembly, an insufficient deposit can be directly related to rework which is undesirable for optimum end of line yields. Even the smallest variations in the process have significant consequences in these applications. Therefore, the entire printing process must be characterized prior to implementation.

Initially Published in the SMTA Proceedings

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